Release time:

2021-05-31

【Summary】In the mold manufacturing process, the forming parts of the mold often require surface polishing treatment. Mastering polishing techniques can improve mold quality and service life, thereby enhancing product quality. Image 1. Mold Polishing Methods and Working Principles Mold polishing usually uses oilstone strips, wool wheels, sandpaper, etc., to cause plastic deformation of the material surface and remove protruding parts of the workpiece to obtain a smooth surface, generally mainly operated manually. For surfaces with high quality requirements, ultra-precision polishing methods can be used. Ultra-precision polishing uses special abrasives, pressed tightly against the surface of the workpiece being processed, and performs high-speed rotational motion in an abrasive slurry. Polishing can achieve a surface roughness of Ra0.008 μm. 2. Common Tools and Specifications for Mold Polishing 1) Common tools for mold polishing include: sandpaper, oilstone, felt wheels, polishing paste, alloy files, diamond grinding needles, bamboo strips, fiber oilstones, and rotary grinding machines. 2) Sandpaper: 150#, 180#, 320#, 400#, 600#, 800#, 1,000#, 1,200#, 1,500#; 3) Oilstone: 120#, 220#, 400#, 600#; 4) Felt wheels: cylindrical, conical, square pointed; 5) Polishing paste: 1# (white) 3# (yellow) 6# (orange) 9# (green) 15# (blue) 25# (brown) 35# (red) 60# (purple); 6) Files: square, round, flat, triangular, and other shapes; 7) Diamond grinding needles: generally 3/32 shank or 1/8 shank, with round wave shape, cylindrical, long straight cylindrical, long conical; 8) Bamboo strips: various shapes suitable for the operator and mold shape, used to press sandpaper and grind on the workpiece to achieve the required surface roughness; 9) Fiber oilstone: 200# (black) 400# (blue) 600# (white) 800# (red) 3. Polishing Process (1) Rough Polishing The surface after precision milling, electrical discharge machining, grinding, etc., can be polished using a rotary surface polishing machine with a speed of 35,000 to 40,000 r/min. Then, manual oilstone grinding is performed, using strip oilstone with kerosene as a lubricant or coolant. The sequence of use is 180#→240#→320#→400#→600#→800#→1,000#. (2) Semi-Precision Polishing Semi-precision polishing mainly uses sandpaper and kerosene. The sandpaper grades are: 400#→600#→800#→1,000#→1,200#→1,500#. In fact, #1,500 sandpaper is only suitable for hardened mold steel (above 52HRC) and not suitable for pre-hardened steel, as this may cause surface damage to pre-hardened steel parts, failing to achieve the expected polishing effect. (3) Precision Polishing Precision polishing mainly uses diamond polishing paste. If polishing cloth wheels mixed with diamond polishing powder or paste are used for grinding, the usual grinding sequence is 9 μm (1,800#)→6 μm (3,000#)→3 μm (8,000#). 9 μm diamond polishing paste and polishing cloth wheels can be used to remove the hair-like scratches left by 1,200# and 1,500# sandpaper. Then, polishing is done with felt and diamond polishing paste, in the sequence of 1 μm (14,000#)→1/2 μm (60,000#)→1/4 μm (100,000#). (4) Working Environment for Polishing The polishing process should be completed in two separate work locations, namely the rough grinding processing location and the precision polishing processing location, and care should be taken to clean the sand particles left on the workpiece surface from the previous process. Generally, after rough polishing is completed with oilstone to 1,200# sandpaper, the workpiece needs to be transferred to a dust-free room for polishing, ensuring that no dust particles adhere to the mold surface. Polishing processes with precision requirements of 1 μm or more (including 1 μm) can be performed in a clean polishing room. If more precise polishing is required, it must be done in an absolutely clean space, as dust, smoke, dandruff, and saliva can all potentially ruin high-precision polished surfaces. After the polishing process is completed, the workpiece surface should be well protected from dust. When the polishing process stops, all abrasives and lubricants should be carefully removed to ensure the workpiece surface is clean, and then a layer of mold rust-proof coating should be sprayed on the workpiece surface. 5. Factors Affecting the Polishing of Mold Surfaces (1) Condition of the Workpiece Surface During the mechanical processing of materials, the surface layer may be damaged due to heat, internal stress, or other factors, and improper cutting parameters can affect the polishing effect. The surface after electrical discharge machining is more difficult to grind than that after mechanical processing or heat treatment, so electrical discharge machining should be followed by electrical discharge finishing; otherwise, a hardened thin layer will form on the surface. If the electrical discharge finishing standard is improperly selected, the depth of the heat-affected layer can reach up to 0.4 mm. The hardness of the hardened thin layer is higher than that of the substrate, and it must be removed. Therefore, it is best to add a rough grinding process to provide a good foundation for polishing. (2) Quality of Steel High-quality steel is a prerequisite for obtaining good polishing quality. Various inclusions and pores in the steel can affect the polishing effect. To achieve good polishing results, the workpiece must specify the surface roughness for polishing at the beginning of mechanical processing. When a workpiece is determined to require mirror polishing, steel with good polishing performance must be selected, and it must be heat-treated; otherwise, the expected effect cannot be achieved. (3) Heat Treatment Process Improper heat treatment can lead to uneven surface hardness or differences in characteristics of the steel, making polishing difficult. (4) Polishing Technology Since polishing is mainly done manually, human skill is still the main factor affecting polishing quality. It is generally believed that polishing technology affects surface roughness; in fact, good polishing technology must also be combined with high-quality steel and correct heat treatment processes to achieve satisfactory polishing results; conversely, if the polishing technology is poor, even the best steel cannot achieve a mirror effect. 6. Precautions for Different Types of Polishing (1) Precautions for Mold Sandpaper Grinding and Oilstone Grinding 1) For the surface of molds with higher hardness, only clean and soft oilstone grinding tools should be used. 2) When changing sandpaper grades during grinding, both the workpiece and the operator's hands must be cleaned to avoid bringing coarse sand particles into the next finer grinding operation. 3) In each grinding process, sandpaper should be used to grind from different 45° directions until the scratches from the previous grade are eliminated. After the scratches from the previous grade are removed, the grinding time must be extended by 25% before switching to the next finer grade. 4) Changing directions during grinding can prevent the workpiece from developing waves or unevenness. (2) Precautions for Diamond Grinding and Polishing Diamond grinding and polishing must be done under as light pressure as possible, especially when polishing pre-hardened steel parts and using fine polishing paste. When polishing with 8,000# polishing paste, the common load is 100 to 200 g/cm2, but it is difficult to maintain the accuracy of this load. To facilitate this, a thin and narrow handle can be made on a wooden strip, or a portion can be cut off a bamboo strip to make it softer. This can help control the polishing pressure to ensure that the pressure on the mold surface does not become too high. When using diamond grinding and polishing, not only must the working surface be clean, but the operator's hands must also be very clean. (3) Precautions for Plastic Mold Polishing The polishing of plastic molds is very different from the surface polishing required in other industries. Strictly speaking, the polishing of plastic molds should be referred to as mirror processing. It not only has high requirements for polishing itself but also has high standards for surface flatness, smoothness, and geometric accuracy. The standards for mirror polishing are divided into four levels: A0=Ra0.008 μm A1=Ra0.016 μm A3=Ra0.032 μm A4=Ra0.063 μm Since methods like electrolytic polishing, fluid polishing, etc., are difficult to precisely control the geometric accuracy of parts, and the surface quality of methods like chemical polishing, ultrasonic polishing, and magnetic grinding polishing does not meet the requirements, the mirror processing of precision molds still relies mainly on mechanical polishing. Precautions during polishing are as follows: 1) When a new mold cavity begins processing, the surface of the workpiece should be checked first, and the surface should be cleaned with kerosene to ensure that the oilstone surface does not stick to dirt, causing it to lose its cutting function. 2) When grinding rough textures, the order should be from difficult to easy, especially for some difficult-to-grind dead corners and deeper bottoms, which should be ground first, followed by the sides and large flat surfaces. 3) Some workpieces may have multiple pieces assembled together for polishing; the rough textures or spark patterns of individual workpieces should be ground separately first, and then all workpieces should be assembled and ground to smoothness. 4) For large flat or side surfaces of workpieces, after grinding rough textures with oilstone, a straight steel plate should be used for light transmission inspection to check for any unevenness or poor conditions such as undercuts. If there are undercuts, it will lead to difficulties in demolding or damage to the workpiece. 5) To prevent mold workpieces from developing undercuts or having some mating surfaces that need protection, saw blades can be attached or sandpaper can be affixed to the edges to achieve ideal protection effects. 6) When grinding mold flat surfaces, pull back and forth, and try to keep the handle of the oilstone as flat as possible, not exceeding 25°, as too steep an angle can cause excessive downward force, leading to many coarse scratches on the workpiece. 7) If the flat surface of the workpiece is polished with a copper or bamboo strip pressing sandpaper, the sandpaper should not exceed the tool area; otherwise, it will grind areas that should not be ground. 8) Try not to use a grinding machine to repair the parting surface, as the grinding wheel head repairs the parting surface relatively coarsely and unevenly. If necessary, the grinding wheel head must be adjusted to achieve concentric balance. 9) The shape of the grinding tool should closely match the surface shape of the mold to ensure that the workpiece is not deformed during grinding. 7. How to Solve Common Problems in Polishing (1) Over-Polishing The biggest problem encountered in daily polishing is "over-polishing," which means that the longer the polishing time, the worse the quality of the mold surface. Over-polishing can manifest in two phenomena: "orange peel" and "pitting." Over-polishing often occurs in mechanical polishing. (2) Causes of "Orange Peel" on Workpieces An irregularly rough surface is referred to as "orange peel." There are many reasons for the formation of "orange peel," the most common being overheating of the mold surface or excessive carbon diffusion, with excessive polishing pressure and prolonged polishing time being the main causes of "orange peel." For example, polishing with a polishing wheel can easily generate heat that causes "orange peel." Harder steel can withstand greater polishing pressure, while relatively softer steel is more prone to over-polishing. Research has shown that the time for over-polishing varies with the hardness of the steel. (3) Measures to Eliminate "Orange Peel" on Workpieces When poor surface quality is detected, many people tend to increase polishing pressure and extend polishing time, which often worsens the surface quality. The following methods can be used for remediation: 1) Remove the defective surface, using a grit slightly coarser than previously used, and then grind, applying less polishing force than before. 2) Stress relief should be performed at a temperature 25 °C below the tempering temperature, using the finest sandpaper for grinding until satisfactory results are achieved, and finally polishing with lighter force. (4) Causes of "Pitting" on Workpiece Surfaces Some non-metallic impurities in the steel, usually hard and brittle oxides, are pulled out from the surface of the steel during polishing, forming micro-pits or pitting. The main factors leading to "pitting" include: 1) Excessive polishing pressure and prolonged polishing time. 2) Insufficient purity of the steel, with a high content of hard impurities. 3) Rust on the mold surface. 4) Black scale not removed. (5) Measures to Eliminate Pitting on Workpieces 1) Carefully re-grind the surface, using a grit slightly coarser than previously used, and perform the final grinding with soft and sharp oilstones before proceeding to the polishing process. 2) When the grain size is less than 1 mm, avoid using the softest polishing tools. 3) Use the shortest polishing time and the least polishing force as much as possible. Polishing of the cavity during mold manufacturing is a very important process, as it relates to the quality and lifespan of the mold and determines the quality of the products. Mastering the principles and processes of polishing, and choosing reasonable polishing methods can improve mold quality and lifespan, thereby enhancing product quality.

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